New methods in parahydrogen hyperpolarised nuclear magnetic resonance spectroscopy

Abstract

The performance of nuclear magnetic resonance (NMR) spectroscopy is widely evaluated in terms of spectral resolution and sensitivity. Significant progress has been made to improve the former criterion, with 60 years of improvements in spectrometer hardware and the development of experiments that control the chemical information content of an NMR spectrum. However, the technique remains comparatively insensitive to competing analytical methods as it relies on transitions between near-degenerate nuclear spin energy levels, whose populations are governed by Boltzmann statistics at thermal equilibrium. Low sensitivity is a particular problem in multiple-pulse NMR experiments where convection, diffusion, and spin relaxation attenuate the measured signal below its theoretical maximum. Manipulation of the populations of nuclear spin energy levels can, in certain cases, lead to an increase of the bulk nuclear spin polarisation beyond that afforded at thermal equilibrium (so-called "hyperpolarisation"). This greater nuclear spin order is typically achieved by external perturbation using species with intrinsically high polarisation, and results in transiently increased NMR signal intensities. One such species is parahydrogen (pH2), the easily prepared nuclear spin singlet isomer of the dihydrogen molecule. Despite its potential to overcome NMR sensitivity limitations, the use of pH2 with multiple-pulse NMR experiments remains underexplored. The aim of this thesis is to describe new methods in pH2 hyperpolarised NMR spectroscopy. Here, pH2 derived hyperpolarisation is used to increase sensitivity in sophisticated multiple-pulse NMR experiments. Chapters 2, 3, and 4 contain introductions to the theoretical NMR background, multiple-pulse NMR experiments, and hyperpolarisation methods necessary for this thesis. Chapter 5 details the infrastructure implemented to conduct pH2 hyperpolarised NMR experiments. Chapters 6 and 7 illustrate how pH2 hyperpolarisation can be exploited to improve sensitivity in single-, and multiple-shot pure shift NMR experiments. Chapter 8 introduces a novel experiment which identifies the bonding networks of spins whose NMR signals are overlapped. Chapter 9 summarises the conclusions drawn from the research introduced in the main body of this thesis and suggests future developments. Chapters 6 and 8 contain published research articles and their supporting information, and are presented without modification. Chapter 7 is presented in the form of a draft manuscript and supporting information that are intended for publication.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mathias Nilsson (Supervisor), Louise Natrajan (Supervisor) & Ralph Adams (Supervisor)